I. Field of the Invention
The present invention generally relates to methods for improving the resistance of plants to attack by disease organisms without adversely effecting plant growth. More specifically, the present invention is directed to methods for improving the resistance of plants to attack by disease organisms including fungi, bacteria and insects by applying an effective amount of an auxin or of another plant growth regulator which will affect the level of auxin in the plant tissue.
II. Description of the Background
Agricultural pesticides are used to control unwanted fungi, bacteria and insect populations. These compounds have allowed the commercial grower to manage the continual attack on his crops by these disease organisms and insects. Similarly, the homeowner and casual gardener have been able to control these pests. Although these traditional chemical applications have been valuable in the past, as environmental concerns have increased, it is unlikely that commercial growers will be able to continue to use pesticides at the same rates in the future. Therefore, improved methods for controlling disease and insect attack by augmenting and stimulating the plant's natural processes of protection are desirable.
Plant hormones have been known and studied for years. Plant hormones may be assigned to one of five categories: auxins, cytokinins, gibberellins, abscisic acid and ethylene. Ethylene has long been associated with fruit ripening and leaf abscission. Abscisic acid causes the formation of winter buds, triggers seed dormancy, controls the opening and closing of stomata and induces leaf senescence. Gibberellins, primarily gibberellic acid, are involved in breaking dormancy in seeds and in the stimulation of cell elongation in stems. Gibberellins are also known to cause dwarf plants to elongate to normal size. Cytokinins, e.g., zeatin, are produced primarily in the roots of plants. Cytokinins stimulate growth of lateral buds lower on the stem, promote cell division and leaf expansion and retard plant aging. Cytokinins also enhance auxin levels by creating new growth from menstematic tissues in which auxins are synthesized. Auxins, primarily indole-3-acetic acid (IAA) promote both cell division and cell elongation, and maintain apical dominance. Auxins also stimulate secondary growth in the vascular cambium, induce the formation of adventitious roots and promote fruit growth.
Auxins and cytokinins have complex interactions. It is known that the ratio of auxin to cytokinin will control the differentiation of cells in tissue cultures. Auxin is synthesized in the shoot apex, while cytokinin is synthesized mostly in the root apex. Thus, the ratio of auxin to cytokinin is normally high in the shoots, while it is low in the roots. If the ratio of auxin to cytokinin is altered by increasing the relative amount of auxin, root growth is stimulated. On the other hand, if the ratio of auxin to cytokinin is altered by increasing the relative amount of cytokinin, shoot growth is stimulated.
The most common naturally occurring auxin is indole-3-acetic acid (IAA). However, other synthetic auxins, including indole-3-butyric acid (IBA); naphthalene acetic acid (NAA); 2,4-dichlorophenoxy acetic acid (2,4-D); and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T or agent orange) are known. While these are recognized as synthetic auxins, it should be acknowledged that IBA does naturally occur in plant tissues. Many of these synthetic auxins have been employed for decades as herbicides, producing accelerated and exaggerated plant growth followed by plant death. Agent orange gained widespread recognition when it was used extensively by the United States Army and Air Force in deforestation applications during the Vietnam War. 2,4-D finds continuing use in a number of commercial herbicides sold for use by the home gardener.
Compounds are classified as auxins based on their biological activity in plants. A primary activity for classification includes simulation of cell growth and elongation. Auxins have been studied since the 1800's. Charles Darwin noticed that grass coleoptiles would grow toward a unidirectional light source. He discovered that the growth response of bending toward the light source occurred in the growth zone below the plant tip, even though it was the tip that perceived the light stimulus. Darwin suggested that a chemical messenger was transported between the plant tip and the growth zone. That messenger was later identified as an auxin.
All plants require a certain ratio of auxin, i.e., IAA, to cytokinin for cell division. While the ratios may vary, it is well known that the ratio of IAA to cytokinin must be much greater for cell division in the apical meristem tissue than the ratio in the meristem tissue of the roots. Each part of a plant may require a different IAA to cytokinin ratio for cell division. For example, different ratios may be required for cell division in the stem, fruit, grain and other plant parts. In fact, it has been estimated that the ratio for apical meristem cell division may be considerably more, in fact, as much as 1000 times greater than the ratio necessary for root cell division. While the mechanism by which this ratio is determined remains unknown, other hormones and enzymes are likely to be involved in its perception.
Plants are most able to resist attack by disease and insects when growing at temperatures from about 68° F. to about 87° F. (about 21-30° C.). In this temperature range it is presumed that plants produce sufficient amounts of auxins, particularly IAA, to maintain normal growth. While ideal temperatures vary among species, crop pants typically grow best in the foregoing range. While temperature is an important factor, it should also be noted that other environmental factors can effect cell division. The moisture content of the plant, the nutrient status (especially the level of available nitrogen), the light intensity on the plant and the age of the plant, together with the temperature, all effect the ability of the plant to produce plant hormones, including IAA and cytokinin which dictate cell division.
As the temperature rises about 90° F. (about 31° C.) or falls below about 68° F. (21° C.) plant growth and cell division slow. Further, susceptibility to attack by disease organisms, including fungi, bacteria and insects, increases. As the temperature further increases above about 90° F. and drops below about 68° F., the production of IAA and other plant hormones decreases at an accelerating rate. Thus, it becomes difficult, if not impossible, to achieve new cell growth at temperatures above about 100° F. Similarly cell growth slows and then ceases as temperatures plunge significantly below about 680 F.
During normal growing conditions with adequate moisture and temperature, i.e., temperatures between about 70° F. and 90° F., the plants will produce an abundance of IAA. The high ratio of IAA to cytokinin and the presence of other hormones inhibit proper cell division in disease microorganisms. Cell division may be further impeded by other inhibitive compounds produced by IAA and other plant hormones. As temperatures increase above about 90° F. or below about 68° F., the ability of plants to produce IAA rapidly diminishes. It is presumed that as IAA production decreases the ratio of IAA to cytokinin decreases to a level where some or all of these microorganisms may multiply and feed in and on the host plant. It should be understood that different microorganisms will require different ratios of IAA to cytokinin to stimulate cell division. Thus, it should be expected that pathological organisms feeding on plant roots require a lower IAA to cytokinin ratio than organisms feeding on the upper parts of the plant. Thus, microorganisms requiring greater levels of IAA may attack the upper plant tissues, e.g., the apical meristem and leaf tissue, where higher IAA levels exist. Similarly, disease organisms requiring lower IAA levels, e.g., soil borne root diseases, may attack the roots where lower IAA levels exist.
When plants are rapidly growing under conditions which include ample moisture, ideal temperatures and ample amounts of nitrogen fertilizer, auxins are efficiently transported out of the tissues where they are metabolized and move downward in the plant. This results in the redistribution of auxin and the reduction of the auxin level in the tissues where it was produced. The result is tissues which are deficient in the level of auxin. These tissues, now dominated by gibberellins, are very susceptible to attack by disease and insects.
All plant diseases are caused by microorganisms. The major microorganisms effecting plant pathological problems are fungi and bacteria. These microorganisms, like the plant, require a certain amount of IAA to carry on cell division. Different microorganisms, like plants, require different amounts of IAA for cell division. Those differences might explain why different microorganisms attack different species of plants and attack different parts of those plants. Such specific attacks may be intended to provide the microorganism with the proper level of IAA to stimulate rapid cell division by feeding on a host plant or portion thereof having the desired IAA concentration. Thus, resistance to such disease organisms may be improved, if the ratio of IAA to cytokinin and other hormones is increased beyond a level sought by the disease organism. Such an increase may be obtained by providing the plant with additional auxin.
By controlling the level of auxins, most often IAA, in plant tissues, the ability of plants to resist attack by both pathogens and pests can be increased. Plant diseases may be controlled by applying to stressed plants additional auxin or other hormones which will affect the auxin level in the tissues. Alternatively, the same results can be achieved by application of other plant hormones which will affect the auxin level in the tissues. For example, the application of cytokinin or other hormones has an affect on regulating the production and/or transport of auxins within the plant. Thus, the application of other plant growth regulators, e.g., cytokinin, can be used to manipulate the level of auxin in the plant. Thus, disease and insect control can be achieved by application of naturally occurring or synthetic auxins or other hormones which will affect the auxin levels without requiring the use of environmentally harmful pesticides.
Those skilled in the art have long sought environmentally friendly methods for improving plant resistance to disease organisms, including both plant pathogens, e.g., fungi and bacteria, and pests, e.g., insects and their larvae. Thus, there has been a long felt, but unfulfilled need for such methods. The present invention solves that need.